The field of the invention is nuclear magnetic resonance methods and systems. More particularly, the invention relates to the RF excitation of spins.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence the magnitude of the net transverse magnetic moment M.sub.t depends primarily on the length of time and the magnitude of the applied excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited spin induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.t. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.0 e.sup.-t/T*.sub.2
The decay constant 1/T*.sub.2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B.sub.1 in a perfectly homogeneous field.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements". Such NMR measurements are divided into a period of RF excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more RF excitation pulses (B.sub.1) of varying magnitude, duration, and direction. Such excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which produces transverse magnetization M.sub.t over a range of resonant frequencies. The prior art is replete with excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
NMR data for constructing images can be collected using one of many available techniques. Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more NMR experiments, each of which comprises at least an RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the resulting NMR signal. As is well known, the NMR signal may be a free indication decay (FID) or, preferably, a spin-echo signal. In a two-dimensional implementation of the well known Fourier transform imaging technique (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the direction of the readout gradient. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
Object motion during the acquisition of NMR image data produces both blurring and "ghosts" in the phase-encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion each view of the NMR signal is acquired in a period short enough that the object may be considered stationary during the acquisition window. In such case the blurring and ghosting is due to the inconsistent appearance of the object from view to view. Motion that changes the appearance between views, such as that produced by patient movement during respiration, the cardiac cycle, or peristalsis, is referred to as "view-to-view motion". Motion may also change the amplitude and phase of the NMR signal as it evolves during the pulse sequence and such motion is referred to as "in-view motion".
There are numerous methods currently employed to reduce motion artifacts and many of these methods require that the motion of the patient be accurately determined. Such motion detection is accomplished, for example, with respirometers which are employed to measure patient respiration, electrocardiograms which are employed to measure motion produced by the cardiac cycle, and position sensors which are employed to measure the motion of a patient's chest. Both blurring and ghosting can be reduced if the data acquisition is synchronized with the functional cycle of the object to reduce view-to-view motion. This method is known as gated NMR scanning, and its objective is to acquire NMR data at the same point during successive functional cycles so that the object "looks" the same in each view.
In U.S. Pat. No. 4,937,526 entitled "Adaptive Method For Reducing Motion and Flow Artifacts In NMR Images", a method is described for correcting the NMR image data set for motion effects after the data has been acquired. This retrospective technique for correcting the acquired image data requires that the patient motion be measured so that the corrective values can be calculated. While physical devices such as a respirometer can be used, the preferred method of motion measurement is to employ the NMR instrument itself. This is accomplished by acquiring "navigator" NMR signals during the scan and analyzing those signals to measure patient motion. These navigator signals are produced throughout the scan and they are non-phase encoded NMR signals which indicate the spin density in a projection along the readout gradient axis. Patient motion is determined by picking out a prominent feature in this projection data and following the motion of that feature along the readout gradient axis. The difficulty with this method is that it is often difficult to accurately pick out the same prominent feature in the NMR projection data during the entire scan. Thus, a method for enhancing the NMR signal produced by this prominent feature would greatly facilitate the use of this navigator NMR signal to correct for motion effects.
Yet another method for reducing image artifacts due to patient motion is to suppress the NMR signal produced by prominent features. For example, in U.S. Pat. No. 4,715,383 a method is described in which the NMR signals produced by spins located outside the region of interest and which might produce motion artifacts in the reconstructed image are suppressed by the application of RF excitation pulses which saturate the spins. Prior to the acquisition of NMR signals from the region of interest, one or more selective RF saturation pulses are applied in the presence of a magnetic field gradient. The RF pulses and the accompanying field gradient are chosen to excite the spins in a slab alongside the region of interest and to thereby saturate them so that they do not produce any significant NMR signal during the subsequent pulse sequence which acquires NMR data from within the region of interest. While this technique is very effective when the offending spins are located outside the region of interest, there is no method for selectively saturating offending spins located within the region of interest without also suppressing the NMR signals needed to reconstruct the image. In other words, there is a need for a method for suppressing the NMR signal produced by prominent features within the field-of-view of the image being produce.